Does the observation function for POMDP always add up to 1?
$O(a, s', z) = \mathbb{P}(z \mid a, s')$ is a conditional probability distribution, so it always needs to sum up to $1$. You should interpret $O(a, s', z)$ as the probability of observation $z$, given that the agent took action $a$ and landed in state $s'$. 

$O(a, s', z)$ is thus not a joint distribution, even though the notation $O(a, s', z)$ might suggest it. In this case, $O(a, s', z)$ simply means that $O$ is a function of $a$, $z$ and $s'$.

If you want to see a proof that conditional probability distributions sum up to 1, have a look at this post.